This proposal will test several new hypotheses concerning the mechanism of protection provided by preconditioning. Because of the proposed central importance of PKC to preconditioning's protection and the controversy surrounding its involvement in larger species such as the dog and pig, the requirement of PKC activation in preconditioning in isolated canine cardiomyocytes will be determined to evaluate whether technical factors or true species differences have heretofore prevented confirmation of the PKC hypothesis in dogs. Recent data reveal that tyrosine kinase activation is also involved in the protection, and that is downstream of PKC, thus forming a kinase cascade. The investigators will test the involvement of several of the tyrosine kinase cascades, especially the p38 MAPK pathway, in ischemic and pharmacologic preconditioning. They will further test the hypothesis that this pathway to protection terminates in inhibition of a protein phosphatase with ultimate preservation of constitutively phosphorylated proteins, possibly cytoskeletal elements.